Possibly missing warning for `unserialize('')` - may be confused with "false"

Start by reproducing the supplied PHP example, then read ext/standard/var.c around php_unserialize_with_options at the linked condition. Check existing unserialize behavior and tests before deciding whether empty input should warn; done means the expected behavior is agreed and covered by an appropriate regression test.

Description

The following code:

<?php

error_reporting(E_ALL);
ini_set('display_errors', '1');
ini_set('display_startup_errors', '1');

$a = 'b:0;'; // serialize(false);
$b = '';

var_dump($a === $b);
var_dump(unserialize($a) === unserialize($b));

Resulted in this output:

bool(false)
bool(true)

But I expected this output instead:

bool(false)

Warning: unserialize(): Empty input in /in/bjBq9 on line 11
bool(true)

Dear Developers, Artists,

Thank you for the marvel, art...

On 2026-09-18, at Libera IRC channel #php, a member ash_worksi raised this issue asking why no warning is raised in PHP, if compared to unserialize(' ');, for example.

My contention is the fact that it doesn't raise a warning like the description says. It's a very narrow edge case, but just like any "non-unserializable" string (...otherwise why would it return false?) it should raise the warning.

unserialize() returning false on it's own does not tell you whether the function failed to unserialize or if the value you unserialized was in fact false.

~ ash_worksi

Later, I tried searching for the actual reason it happens, and the condition involved is likely the following, in the function php_unserialize_with_options:

https://github.com/php/php-src/blob/13cec7c276033883e1e0ee22ea92fa1838b06dca/ext/standard/var.c#L1414-L1416

The condition explicitly checks if the string is empty, and returns "false" by design, and considering the first commits checked, it has been so for at least "20 years ago".

I believe that for so much time, it was discussed already, but just in case, is it still actually expected?

Best and kind regards

PHP Version

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